D.P.
Your post is confusing because it has NOTHING to do with the title!
Here are some thoughts:
1. You seem to feel persecuted by a woman that you've never met? Why is that? A new "mom" is something important going on in HER daughter's life. I would be suspicious of a mom who is not concerned about that.
2. You write that you "are at the five month mark and we feel very sure that we are serious" What does that mean? Serious about liking each other, seriously in love, serious enough to give dating another year? Serious enough to live together? Serious enough to get married?
If you're not getting married, I might not want that environment for my kid either. Like it or not, he has a 7 yo DAUGHTER who looks to dad to see the proper treatment of women. How will your living arrangement fit in with that?
3. What's the rush? If he is "serious" and "the O." what's another 6 mos or a year for the daughter to get to know you a little? In the scheme of life, that's a heartbeat.
4. Don't pre-judge his ex the way you feel she is pre-judging you. If I were you I would make this woman by new BFF right away. You know the drill, stepmother=responsibility. You both need to get on the same page with this precious child. Yes, I think with a common goal, a woman and an ex can have a good relationship. You don't have to "like" her, nor her, you. But if you can be friendly, communicative and trust each other that will be huge!
5. I doubt she can keep the daughter away from her ex just because she doesn't like you.
6. This man has at least another 10-11 year of responsibility (financial, emotional, psychological, etc.). Know that going in. He is correct if he puts her BEFORE you. If he doesn't, I'd run in the other direction.
My opinion, if your plan is to "move in together" and attempt to adjust to a.) baby mama drama, b.) your daughter moving back in and c.) living with a man you've dated for 5 months, I'd say you're crazy.
Waaaaaay more time needs to be invested and all of the above steps need to be done over time, not all at once. Your daughter and his daughter need to remain the #1 priorities right now.
If the plan is engagement, wedding, moving everyone under O. roof, I think it needs to bve done very carefully and not rushed.
JMO. Good luck!